1. Field of the Invention
The present invention relates to an inverse-time relay for use in protection of power transmission and distribution lines.
2. Description of the Prior Art
FIG. 4 is a circuit diagram of input circuit portion for showing a prior art inverse-time relay, in which 1 denotes a current transformer outputting secondary current proportional to the current in the line to be protected, 2 denotes a rectifying-smoothing-settling portion for current transformation, current-voltage conversion, rectification and smoothing of the output current from the current transformer 1, 3 denotes level detection portion for detecting whether or not the output voltage value from the rectifying-smoothing-settling portion 2 is in excess of a specified detection level, 4 denotes an operating-time timer of the inverse time type for producing a timing output after a delay which varies inversely with input current, 5 denotes a control power source for the electronic circuit, and 6 denotes a switch for supplying the operating-time timer 4 with an input voltage in response to the level detection output from the level detection portion 3.
Operations of the relay will be described in the following. First, the current input from the secondary side of the current transformer 1 is transformed into a suitable current value by the rectifying-smoothings-settling portion 2, and then, converted by current-voltage conversion into an a.c. voltage of the magnitude corresponding to the tap setting. The a.c. voltage is rectified and smoothed into a d.c. voltage Vi. Since d.c. voltage Vi is proportional to the input current, it will be referred to as "input voltage Vi" in this specification. This voltage Vi is compared with a reference voltage in the level detection portion 3 and a decision of level as to whether or not it is higher than the reference voltage is made therein. When the input voltage exceeds the reference voltage, the switch 6 is turned ON, whereby the input voltage Vi is applied to the operating-time timer 4. The operating-time timer 4 exhibits the inverse-time characteristic to vary the operating time according to the magnitude of the input voltage Vi and operates so as to deliver a decision output when a predetermined operating time has elapsed.
In the inverse-time relay of the prior art, as described above, the range within which the magnitude of the current input could correctly be detected was up to an input voltage Vi not exceeding the power source voltage, and so, it was unable to enlarge the dynamic range. Hence, in an overcurrent relay exhibiting an inverse-time characteristic responding to detected magnitude of the input, for example, because of the small dynamic range for the input, the operating time was saturated as indicated in FIG. 5 by the curve T1 when the input current value exceeded a certain point. Thus, there was such a problem in the prior art that the desired operating-time characteristic as indicated by the curve T2 was unable to be provided.